One of the major problems faced in telephone communications is to maintain the singing return loss on the telephone loop at an acceptable level. This problem is particularly aggravated when long distance communications are encountered where the circuit switches from a two-wire to four-wire configuration utilizing a hybrid network. The various terminating impedances and loop lengths encountered on the two-wire side of the system make it impossible to provide a good match across the whole audio spectrum utilizing a single balance impedance on the hybrid network. The result is that the singing return loss (i.e. the amount of signal coupled between the input and output of the four-wire path) varies not only with frequency but also with the length of the two-wire line, any inductive loading applied to the line to compensate for high frequency roll-off, and the terminating impedance (e.g. the telephone) at the far end of the loop. In general a margin of 4 dB or more singing return loss (SRL) is required for all loops. The SRL is defined as the minimum return loss (RL(f)) for a given loop and balance network over the frequency range where: EQU RL(f)=T(f)-T(s)
where T(f) is the transhybrid loss at frequency f (i.e. the ratio of the output/input signals at the four-wire port (in dB) at a frequency f); and T(s) is the transhybrid loss at 1000 Hz with the hybrid short-circuited at the two-wire port.
Various solutions have been proposed for this problem. These include multiple balance impedances for the hybrid network; voice switched loss in the four-wire path; the introduction of fixed loss in the four-wire path; and echo cancellation. Those solutions such as introducing fixed attenuation in the four-wire path improve the return loss at the expense of lower speech signal levels and consequently are not considered very satisfactory. Other solutions such as echo cancellation can provide very good results, however at a price which is usually too high to justify implementation on a per-line basis. As a result, these techniques are only utilized at the present time in specialized applications such as overseas communications by satellite. Thus, any solution must not only be technically acceptable but must also be cost effective in order to justify implementation on a line by line basis.
As mentioned above, one solution is to select one of a plurality of balance impedances for the hybrid in order to provide a better match with a particular two-wire loop impedance. However, in order to do this, it is necessary that there be some indication of the impedance of the two-wire loop in order to determine which balance impedance is to be utilized. To be effective, this must be done on a periodic basis, normally at the commencement of each telephone call. One procedure is to first disconnect the two-wire line from the hybrid and then measure the line impedance, so that the correct balance impedance can be connected to the hybrid network. However, this solution is generally expensive to implement and not acceptable to the telephone operating companies.
Telephone lines (i.e. those between the central office and the terminating set) are generally divided into two major groups, those that are inductively loaded utilizing loading coils, (or which exhibit such characteristics) and those that are nonloaded (or exhibit nonloaded characteristics). It has been found that a relatively simple method of determining either of these conditions is to measure the phase delay of a return signal on the four-wire output of the hybrid network which was applied to the input. It has also been found that this test yields sufficient information to control the selection of the correct balance impedance or network to be utilized.